The invention relates to an overvoltage protecting element of a gaseous discharge tube type which improves a time lag on the commencement of a discharge in response to the application of an overvoltage.
An overvoltage protecting element of the type described involves a time lag in its discharge when an overvoltage is applied thereto, thereby giving rise to a residual voltage. Efforts have been directed heretofore at minimizing such time lag as by enclosing radioisotope in gaseous form in a discharge element or mixing it with a suitable binder to be disposed on the inner wall of the discharge element. However, the attempt at minimizing the time lag in the discharge of the element through the use of such radioisotope failed to provide a satisfactory result in that (1) an increase in the amount of radioisotope added results in a saturation of the intended effect; (2) the amount of radioisotope which can be enclosed in a single discharge element is constrained by the law regulation, and a variety of security and maintenance requirements are imposed on the manufacturing equipment; (3) a repeated discharge operation causes the electrode material to be sputtered onto the inner wall of the discharge element to be deposited over the radioisotope applied, thus reducing its effect; (4) the effect of the radioisotope decreases according to its half-life; and (5 ) there is a possibility of pollution.
As an alternative to the use of the radioisotope, U.S. Pat. No. 3,588,576 discloses a spark-gap device comprising a pair of main electrodes, a tube of an insulating material such as ceramic or glass which holds the main electrodes in position and establishes a discharge space which is isolated from the environment and which is filled with a gas, and a narrow conductive layer disposed on the inner surface of the tube in electrical contact with one of the main electrodes and extending toward the other electrode. A region of concentrated potential gradient is formed on the surface of the insulating material to cause a creeping discharge from the edge of the layer along the inner wall of the tube which produces an ionization effect, thereby promoting a discharge across the main electrodes. In this manner, the discharge surface of the main electrodes are bombarded with ions. However, in an overvoltage protecting element of this type, if a discharge of an increased current frequently occurs across the pair of main electrodes, the electrode material will be sputtered to form a thin annular conductive layer on the inner wall of the tube around the electrodes, and thin conductors may communicate with each other to cause a degradation in the insulating characteristic of the element.